---
title: "Earthing Cable Size Calculator: AS/NZS 3000 Table 5.1 Earth Conductor Sizing"
url: "https://scaleset.com.au/calculator/earth-cable-size"
description: "Free AS/NZS 3000 Table 5.1 earthing cable size calculator: the minimum protective earth for copper or aluminium actives, parallel runs and the fault check."
updated: "2026-10-13"
standards:
  - "AS/NZS 3000 Table 5.1"
  - "AS/NZS 3000 Clause 5.3.3.1.3"
  - "AS/NZS 3008.1.1 Table 5.1"
author: "Wisam Tozah (B.Eng (Electrical), MIEAust, CPEng, NER, NSW DBP, NSW PRE, APEC, IntPE(Aus))"
site: "ScaleSet"
license: "https://scaleset.com.au/terms"
usage: "Cite and link. Do not reproduce or train on this content."
---

# Earthing Cable Size Calculator: AS/NZS 3000 Table 5.1 Earth Conductor Sizing

Source: https://scaleset.com.au/calculator/earth-cable-size

Determine minimum protective earthing conductor sizes using AS/NZS 3000 Table 5.1 criteria.

## Key facts

- Minimum earth conductor sizes come from AS/NZS 3000 Table 5.1, stepping with the active conductor size.
- From 16 mm² to 300 mm² the copper earth is 24 to 40 percent of the active area (6 mm² for a 25 mm² active, 25 mm² for 70 mm², 120 mm² for 300 mm²), and it stays at 120 mm² for actives of 300 mm² to 630 mm².
- Parallel actives are read as their summed area per phase (AS/NZS 3000 Clause 5.3.3.1.2): 2 × 240 mm² reads AS/NZS 3000 Table 5.1 at the next size up from 480 mm² and takes 120 mm² for the group, not the 95 mm² a single 240 mm² gets. With an earth in each set, ScaleSet holds the earths together to that 120 mm² and each to at least the 95 mm² of its own cable, so 95 mm² in each. Above 630 mm² summed, the last row of AS/NZS 3000 Table 5.1 takes 25% of the sum.
- The earth must also pass the adiabatic short-circuit check of AS/NZS 3000 Clause 5.3.3.1.3, S = √(I²t) / k, which can force a larger size than AS/NZS 3000 Table 5.1.
- Switch the fault check on, enter the earth-fault current and the device clearing time, and the calculator runs that check for you on each earth conductor. k comes from AS/NZS 3008.1.1 Table 5.1 by where the earth starts the fault: 111.2 for thermoplastic (PVC) and 142.9 for XLPE or EPR when it is a core of the cable or a separate earth laid with the loaded actives, which sit at their operating temperature; 135.9 and 170.1 for an earth run apart from loaded cables, at the 40 °C ambient. The larger of the table and fault-check answers governs. Cable Selection and EleCAD use the same figures.

## Who this page is for

Designers checking protective earthing conductor sizing against active conductor and device conditions.

## Relevant standards

- AS/NZS 3000 Table 5.1
- AS/NZS 3000 Clause 5.3.3.1.3
- AS/NZS 3008.1.1 Table 5.1

## What this tool does

- Look up protective earthing conductor sizes from active conductor data.
- Use the result during final circuit design and compliance review.
- Connect earthing outcomes with cable sizing and demand calculations.

## How to size an earth conductor under AS/NZS 3000:2018

1. **Pick the active conductor material** — Choose copper or aluminium for the active. AS/NZS 3000 Table 5.1 has a column for each, and the aluminium column starts at 16 mm². The earthing conductor is copper.
2. **Enter the active size and parallel runs** — Pick the largest active conductor supplying the part of the installation the earth protects, and the number of cables in parallel per phase. Parallel actives are read as their summed area (AS/NZS 3000 Clause 5.3.3.1.2). Say whether the earth runs separately (2.5 mm² minimum) or as a core of a multi-core cable, and, for parallel runs, whether there is an earth in each set or one for the group.
3. **Switch on the fault check** — Optional. Enter the r.m.s. earth-fault current through the protective device in kA and its disconnecting time in seconds (0.1 s to 5 s). These drive the AS/NZS 3000 Clause 5.3.3.1.3 adiabatic check on each earth conductor. Choose Table + fault check (the larger of the two) or Calculation only (AS/NZS 3000 Clause 5.3.3.1.2(b)).
4. **Pick the earth insulation** — A core of the cable, or a separate earth laid with the loaded actives, starts a fault at their operating temperature (k = 111.2 PVC, 142.9 XLPE or EPR); an earth run apart from loaded cables starts at the 40 °C ambient (k = 135.9 PVC, 170.1 XLPE or EPR). Both sets are AS/NZS 3008.1.1 Table 5.1.
5. **Review the governing result** — The calculator shows the AS/NZS 3000 Table 5.1 figure, the per-set figure for parallel runs and the Equation 5.1 result, and names what governs. Export the PDF report citing both (a free account to download).

## Earth cable sizing under AS/NZS 3000:2018 Table 5.1

### What AS/NZS 3000 Table 5.1 specifies

AS/NZS 3000:2018 Table 5.1 specifies the minimum copper earthing conductor as a function of the largest active conductor supplying the part of the installation being protected (AS/NZS 3000 Clause 5.3.3.1.2). The copper earth equals the active up to 2.5 mm², holds at 2.5 mm² for 4 mm² and 6 mm² actives, then falls behind: 4 mm² for a 10 mm² active, 6 mm² for 16 mm² and 25 mm², 10 mm² for 35 mm², 16 mm² for 50 mm², 25 mm² for 70 mm² and 95 mm², and so on up to 120 mm², which serves every active from 300 mm² to 630 mm². An aluminium active reads its own column, which is the same size at 25 mm² and 630 mm² and one to two sizes smaller everywhere else.

AS/NZS 3000 Table 5.1 is a deemed-to-comply minimum. It does not account for the prospective fault current or the clearing time of the device: for that, AS/NZS 3000 Clause 5.3.3.1.3 gives the adiabatic calculation, Equation 5.1, as the alternative to the table.

### When the AS/NZS 3000 Clause 5.3.3.1.3 adiabatic check supersedes AS/NZS 3000 Table 5.1

Where the earth-fault current through the protective device, held for the time the device takes to clear it, carries more energy than the AS/NZS 3000 Table 5.1 size can absorb without its insulation passing its short-circuit temperature limit, the table size is not enough. Equation 5.1 of AS/NZS 3000 Clause 5.3.3.1.3 gives S = √(I²t) / k, where I is the r.m.s. fault current through the device, t the device’s disconnecting time for that current (the equation applies only from 0.1 s to 5 s) and k a constant for the conductor material and its initial and final temperatures, which AS/NZS 3000 refers to AS/NZS 3008.1.1.

k is where most hand calculations go wrong. AS/NZS 3008.1.1 Table 5.1 tabulates it by the temperature the conductor starts the fault at. An earth core of the same multicore cable lies against loaded actives and starts at their operating temperature, 75 °C for PVC and 90 °C for XLPE, which gives 111.2 and 142.9 respectively, the figures AS/NZS 3008.1.1 uses in its own worked example A.9. A separate earth laid with the loaded actives it serves, in the same conduit, tray or group, sits at much the same temperature, so ScaleSet gives it the same k: the safe side. Only an earth run apart from loaded cables starts at the ambient, 40 °C for cables in air under AS/NZS 3008.1.1: k = 135.9 for PVC (to 160 °C) and 170.1 for XLPE or EPR (to 250 °C, AS/NZS 3008.1.1 Table 5.2). AS/NZS 3000 gives that figure in Note 1 to Clause 5.3.3.1.3: K = 136 for a copper conductor with PVC insulation not laid up with other conductors. Using the 40 °C values on an earth that starts at 75 °C over-rates it by about 20 percent.

The ScaleSet calculator applies both AS/NZS 3000 Table 5.1 and Equation 5.1 with the k for the run you pick, takes the larger, and says which governed. It can also size by calculation alone, as AS/NZS 3000 Clause 5.3.3.1.2(b) allows, held to the AS/NZS 3000 Clause 5.3.3.4 minimum; that answer can be below the table, and the earth fault-loop impedance then has to be verified separately.

### Aluminium actives, parallel runs and the main earthing conductor

Where the active conductor is aluminium, AS/NZS 3000 Table 5.1 has its own column, starting at 16 mm²: the copper earth for a 70 mm² aluminium active is 10 mm² where a 70 mm² copper active needs 25 mm². AS/NZS 3000 Table 5.1 gives copper earths only. An aluminium earthing conductor, which AS/NZS 3000 Clause 5.3.2.1.2 permits under conditions, has no table of its own; Equation 5.1 takes K for the conductor’s material, and the aluminium K of AS/NZS 3008.1.1 Table 5.1 (90.1 for PVC and 112.7 for XLPE from 40 °C) gives about one and a half times the copper area.

Parallel actives are treated as one conductor of their summed cross-sectional area (AS/NZS 3000 Clause 5.3.3.1.2), so a submain of two 240 mm² copper cables per phase reads AS/NZS 3000 Table 5.1 at 480 mm² (the 500 mm² row) and takes a 120 mm² earth for the group. The standard does not say how that figure is shared when each set carries its own earth, as with a 4C+E cable per set. ScaleSet holds the earths together to the group figure (120 mm² over two, 60 mm² each, so 70 mm²) and each to at least the AS/NZS 3000 Table 5.1 earth for its own cable on its own (95 mm² for 240 mm²), so 95 mm² in each. Three 240 mm² cables per phase are 720 mm², whose 25% is 180 mm², so 185 mm² for the group and 95 mm² in each cable. The short-circuit check is applied to each earth on its own with the whole fault current, because a fault near the start of one set returns down that set’s earth almost alone. Where one earth serves a group so large that its area passes 630 mm², it is run as the fewest equal parallel conductors that reach it. Cables in parallel are not less than 4 mm² (AS/NZS 3000 Clause 3.4.3(a)). A smaller earth can still be justified by calculation (AS/NZS 3000 Clause 5.3.3.1.2(b)), with the earth fault-loop impedance verified as well. Enter the conductor size, the number of parallel runs and the earth arrangement and the calculator does both steps; above 630 mm² summed, AS/NZS 3000 Table 5.1 takes 25% of the sum, so four 240 mm² cables per phase (960 mm²) need a 240 mm² earth. The Cable Selection calculator reads AS/NZS 3000 Table 5.1 the same way for a parallel run.

The main earthing conductor to the electrode is also read from AS/NZS 3000 Table 5.1, against the consumer mains; in copper it is not less than 4 mm² and need not be greater than 120 mm² (AS/NZS 3000 Clause 5.3.3.2). The MEN connection is at least the current-carrying capacity of the main neutral or, on a switchboard rated 800 A or more per phase, sized from AS/NZS 3000 Table 5.1 or by calculation as a protective earthing conductor (AS/NZS 3000 Clause 5.3.5.2).

### AS/NZS 3000 Table 5.1 at a glance

Note the shape of the copper column: it tracks the active size up to 2.5 mm², then falls behind it, and above 300 mm² it stops rising at 120 mm² altogether. Beyond 630 mm² the table’s last row takes 25% of the active size, in both columns.

Read this as a floor rather than an answer. AS/NZS 3000 Table 5.1 is a deemed-to-comply minimum based on the active conductor size alone: it knows nothing about the prospective fault current at the point of installation or how long the protective device takes to clear it, which are the two quantities that actually determine whether the earth survives a fault.

Always confirm the tabulated size against the adiabatic check, and against the earth fault loop impedance the circuit needs to achieve its required disconnection time. Both can force a larger conductor than the table asks for.

### Why the earth is often larger than AS/NZS 3000 Table 5.1 requires

Three things commonly push the protective earthing conductor above the tabulated minimum. The first is the adiabatic check: on an installation with a high prospective fault current, or a device with a slow clearing time, I²t can exceed k²S² for the AS/NZS 3000 Table 5.1 size and force the next size up.

The second is earth fault loop impedance. The loop includes the earthing conductor, so on a long circuit a larger earth lowers Zs, raises the fault current and brings the disconnection time back inside the AS/NZS 3000 limit. Where a circuit only just fails its Zs check, increasing the earth is often the cheaper remedy than increasing the active.

The third is simply the installation: a combined active-and-earth cable is supplied with a fixed earth core, so the available size is whatever the manufacturer builds. Where that core is smaller than the checks require, the cable has to change rather than the earth.

## Key terms

- **Protective earthing conductor (PE)** — The conductor connecting exposed conductive parts to the main earthing terminal, so a fault current has a path back to the source and the protective device operates.
- **AS/NZS 3000 Table 5.1** — The AS/NZS 3000 table giving the minimum protective earthing conductor size for a given active conductor cross-sectional area. It is a minimum, not necessarily a sufficient size.
- **Adiabatic equation (I²t ≤ k²S²)** — The short-circuit thermal withstand check: the fault energy the conductor must survive against what its cross-section can absorb before its insulation is damaged. It assumes no heat escapes during the fault, which is why it is called adiabatic.
- **k factor** — The constant in the adiabatic equation, set by the conductor material and by the temperature the conductor starts and finishes a fault at. AS/NZS 3008.1.1 Table 5.1 tabulates it: for copper with PVC, 111.2 from 75 °C (a core of the cable, or an earth laid with the loaded actives) or 135.9 from 40 °C (an earth run apart from loaded cables); for copper with XLPE, 142.9 or 170.1.
- **Earth fault loop impedance (Zs)** — The total impedance of the fault current path, including the earthing conductor. A larger earth conductor lowers Zs, which raises the fault current and shortens the disconnection time.
- **MEN system** — Multiple Earthed Neutral, the Australian earthing arrangement in which the neutral is earthed at the supply and again at each installation.

## Frequently asked questions

### How do I work out the earth cable size for a circuit?

Read AS/NZS 3000:2018 Table 5.1 against the largest active conductor supplying the circuit, or calculate the size with AS/NZS 3000 Equation 5.1 of Clause 5.3.3.1.3; AS/NZS 3000 Clause 5.3.3.1.2 allows either. AS/NZS 3000 Table 5.1 always gives a copper earth: the same size as the active up to 2.5 mm², then smaller, such as 2.5 mm² for a 6 mm² active and 4 mm² for 10 mm². A separate single-core earthing conductor must be at least 2.5 mm²; an earth core inside a sheathed multi-core cable can be as small as 1 mm² (AS/NZS 3000 Clause 5.3.3.4). Pick the active size and this calculator reads the table; add the fault current and clearing time and it runs the equation too and gives the larger.

### Can the earthing conductor be smaller than the active?

Yes, and by a long way. From 4 mm² actives upward AS/NZS 3000 Table 5.1 already allows a smaller earth (a 4 mm² active takes a 2.5 mm² earth), and from 16 mm² to 300 mm² the earth is 24 to 40 percent of the active: 50 mm² takes 16 mm², 95 mm² takes 25 mm², 240 mm² takes 95 mm². Above that it stays at 120 mm² up to a 630 mm² active. AS/NZS 3000 Table 5.1 is only the minimum. Where fault current is high and the protection is slow, also check the earth against the adiabatic equation in AS/NZS 3000 Clause 5.3.3.1.3, which can call for a bigger conductor.

### Does the earthing conductor need to be the same material as the active?

No. AS/NZS 3000 Table 5.1 always gives you a copper earth size: its two columns tell you whether the active you are pairing it with is copper or aluminium, and an aluminium active needs the same or a smaller copper earth than a copper active of the same size. Aluminium earthing conductors are allowed under AS/NZS 3000 Clause 5.3.2.1.2, but with conditions: sizes up to 10 mm² must be solid, main earthing conductors must be at least 16 mm², and they must not run underground or in damp situations unless designed and suitable for it.

### What size is the main earthing conductor under AS/NZS 3000?

Read AS/NZS 3000 Table 5.1 against the largest active conductor of the consumer mains, then hold a copper main earthing conductor to at least 4 mm²; it need not be larger than 120 mm² (AS/NZS 3000 Clause 5.3.3.2, which also lists exceptions). So 16 mm² copper consumer mains take a 6 mm² main earth, and 6 mm² mains take 4 mm², not the 2.5 mm² the table alone gives. Enter the consumer mains active size here for the AS/NZS 3000 Table 5.1 figure and apply the 4 mm² floor. An aluminium main earthing conductor is 16 mm² at least (AS/NZS 3000 Clause 5.3.2.1.2).

### What size earth cable do I need for a 25 mm² active under AS/NZS 3000?

AS/NZS 3000 Table 5.1 requires a 6 mm² copper earth for a 25 mm² copper active (and also 6 mm² for a 25 mm² aluminium active). AS/NZS 3000 Table 5.1 is only the minimum: on submains with high fault current and slow protection you should also check the earth against the adiabatic equation in AS/NZS 3000 Clause 5.3.3.1.3, which can call for a bigger conductor. Enter the fault current and clearing time and this calculator runs that check beside the table.

### When does the AS/NZS 3000 Clause 5.3.3.1.3 adiabatic check govern over AS/NZS 3000 Table 5.1?

When the energy of the earth fault, the r.m.s. fault current through the device squared times the device’s disconnecting time for that current, needs more copper than the table gives: S = √(I²t) / k comes out above the AS/NZS 3000 Table 5.1 size. Equation 5.1 applies only to disconnection times of 0.1 s to 5 s (AS/NZS 3000 Clause 5.3.3.1.3). Switch the fault check on and the calculator works it out on each earth conductor and says which governs.

### How is the earth sized for cables in parallel?

AS/NZS 3000 Clause 5.3.3.1.2 reads AS/NZS 3000 Table 5.1 against the summed area of the parallel actives, at the nearest larger tabulated size, and that figure is the earth for the whole group. Three 240 mm² cables per phase are 720 mm², above the 630 mm² row, so the table takes 25%: 180 mm², rounded up to a 185 mm² earth for the group. With one earth for the group that is the size. With an earth in each set, as with 4C+E cables, the standard does not say how the 185 mm² is shared; ScaleSet holds the three earths together to 185 mm² and each to at least the 95 mm² a single 240 mm² cable takes, so 95 mm² in each (285 mm² together). The Cable Selection calculator and EleCAD let you choose an earth in each set or one earth cable for all; on Auto they take one cable for all on single-core sets, where it passes the fault and loop checks. Each earth is still checked for the whole fault current on its own, and cables in parallel are not less than 4 mm² (AS/NZS 3000 Clause 3.4.3(a)).

### Can the earth be smaller than AS/NZS 3000 Table 5.1?

Yes, by calculation. AS/NZS 3000 Clause 5.3.3.1.2 sizes the earth either from Table 5.1 or by calculation to Clause 5.3.3.1.3, S = √(I²t) / k, for disconnection times of 0.1 s to 5 s. The calculated size is held to the AS/NZS 3000 Clause 5.3.3.4 minimum: 2.5 mm² for a separate single-core earth, 1 mm² for a core of a sheathed multi-core cable. It must also give an earth fault-loop impedance low enough to disconnect in time (AS/NZS 3000 Clause 5.3.3.1.1(b) and Clause 5.7), so verify the loop before relying on it. Choose Calculation only in the fault check and the calculator works it out and warns when the answer is below the table.

### How do I run the short-circuit check on the earth conductor?

Enter the r.m.s. earth-fault current through the protective device (in kA) and its clearing time (in seconds, 0.1 s to 5 s), say whether the earth is laid with the actives, run apart from them or a core of the same cable, and pick its insulation. The calculator works S = √(I²t) / k, rounds up to the next standard copper size and compares it with the AS/NZS 3000 Table 5.1 figure; whichever is larger is the size to install. k is from AS/NZS 3008.1.1 Table 5.1, to the limits of AS/NZS 3008.1.1 Table 5.2: for a core of the cable or an earth laid with the loaded actives it is 111.2 for PVC (from 75 °C, 160 °C final) and 142.9 for XLPE or EPR (from 90 °C, 250 °C); for an earth run apart from loaded cables it is 135.9 and 170.1 (from the 40 °C ambient). A PVC earth above 300 mm² is held to 140 °C, which lowers k again. A 10 kA fault cleared in 1 s on a PVC earth laid with the actives needs 89.9 mm², so a 95 mm² earth, where AS/NZS 3000 Table 5.1 would have allowed 25 mm² on a 70 mm² active; that is AS/NZS 3008.1.1 Example A.9. Run apart, the same fault needs 73.6 mm², also 95 mm².

### Why does k depend on where the earth runs?

The adiabatic equation assumes all the fault energy goes into heating the conductor from its initial temperature to the insulation limit. An earth run apart from loaded cables carries no load and sits at the ambient, 40 °C under AS/NZS 3008.1.1, so a PVC earth has 120 °C of headroom to absorb the fault (k = 135.9). A core of a multicore cable, or a separate earth laid with the loaded actives, sits at their 75 °C, so it has only 85 °C of headroom (k = 111.2) and comes out about 22 percent larger for the same fault. AS/NZS 3008.1.1 Table 5.1 gives both rows. ScaleSet takes the 75 °C figure for any earth laid with the actives, the safe side, in this calculator, Cable Selection and EleCAD alike.

### Can the earth be smaller than the neutral, and how is the MEN connection sized?

Yes. AS/NZS 3000 Table 5.1 sizes the protective earth from the active conductor, not the neutral, so a 95 mm² copper submain with a full-size neutral takes a 25 mm² earth. The neutral carries load current; the earth carries fault current until the protection opens. Neutral and earth normally meet at the MEN connection on the main switchboard (AS/NZS 3000 Clause 5.3.5.1 lists where else the connection may be made), which AS/NZS 3000 Clause 5.3.5.2 sizes to at least the current-carrying capacity of the main neutral, or, on a switchboard rated 800 A or more per phase, from AS/NZS 3000 Table 5.1 or by calculation as for a protective earthing conductor (with exceptions where it need not exceed the main earthing conductor).

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